re PR preprocessor/16192 (Bug in expression evaluation when operand is missing)

	* doc/cpp.texi: Don't document what we do for ill-formed expressions.
	* doc/cppopts.texi: Clarify processing of command-line defines.

libcpp:
	PR preprocessor/16192
	PR preprocessor/15913
	PR preprocessor/15572
	* expr.c (_cpp_parse_expr): Handle remaining cases where an
	expression is missing.
	* init.c (post_options): Traditional cpp doesn't do // comments.

testsuite:
	* gcc.dg/cpp/if-mop.c: Two new testcases.
	* gcc.dg/cpp/trad/comment-3.c: New.

From-SVN: r84080